Inside one particular warehouse in suburban Denver, it feels like Antarctica. In a sense, it is.

Within the cavernous walls of polystyrene foam lies an 80,000-cubic-foot deep freeze, filled with columns of ice: a few from Wyoming’s Wind River Range, many more from Greenland, the majority from Antarctica. The cores, each about as wide as a roll of French bread and up to a meter and a half long, are stored in aluminized cardboard tubes and stacked on metal shelves that reach from floor to ceiling. Whirring fans blast air across refrigerated coils, keeping the air inside the freezer 35 degrees below zero Fahrenheit.

"It’s as warm as it gets during wintertime in Antarctica," explains Eric Cravens, an assistant curator at the National Ice Core Laboratory.

The laboratory houses what the U.S. Geological Survey claims is the "largest and most comprehensive collection of polar ice cores in the world." Although the cores require great skill to interpret, they can provide an "incredibly detailed record of snowfall that has been converted to ice," says David Naftz, a Geological Survey research hydrologist who has supervised the retrieval of ice from a glacier in the Wind River Range.

When layers of snow are buried under new snowfall, they eventually solidify into glacial ice; the resulting layers can, like tree rings, provide a year-by-year record of past climatic conditions.

Paleoclimatologists — experts in ancient climates — study the layers for dust from volcanic eruptions, forest fires or dust storms, and for pollen from plants that grew at the time. They also analyze the chemical composition of air bubbles trapped in the cores.

Data from ice cores can indicate past precipitation and temperature shifts, and supply evidence of changes in circulation patterns and surface temperatures in nearby oceans. In Greenlandic ice, scientists have even found low levels of lead that coincide with the first smelters in the Roman Empire, 2,000 years ago. The content of air bubbles in ice cores shows that atmospheric levels of carbon dioxide and other greenhouse gases have increased by some 30 percent since the Industrial Age began about 250 years ago.

Layers in ice cores tend to be less distinct than tree rings, and since glaciers are found only in the planet’s coldest places, cores are only indirect evidence of what happened on warmer continents. Where ice cores shine is in the length of their record. Ice cores from the Wind River Range — the only reliable glacial ice in the American West — go back only 300 years, but ice cores from Greenland reach 125,000 years into the past. In Antarctica, new drilling completed late last year will extend the ice-core record to 900,000 years.

"Ice cores provide a bigger but foggier crystal ball than the tree rings," says James White, director of the Environmental Studies Program at the University of Colorado in Boulder.

This long view puts modern climate change into startling — and disquieting — context. Ice cores from Antarctica reveal that during the last 430,000 years, the world’s climate has been as warm as it is today only 5 percent to 10 percent of the time. Lulls between glaciers in North America have generally lasted only 4,000 to 6,000 years. In contrast, our current lull has lasted 12,000 years.

"Our current climate appears to be a rarity, if not an anomaly," White wrote in the journal Science last summer.

The ice-core record also illuminates the climate’s inherently fickle nature, showing that localized average temperatures have risen 15 to 18 degrees within just a decade, and can cool nearly as fast. Many scientists worry that the loading of the atmosphere with greenhouse gases could cause the climate to lurch like an overloaded clothes washer. Whether we would be launched into another ice age or something altogether different is among the many scientific unknowns.

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